Voice frequency signaling circuit



June 16, 1953 w. w. FRITSCHI ETAL 2,642,500

VOICE FREQUENCY SIGNALING CIRCUIT Filed May 1, 1952 5 Sheets-Sheet 2 PERM SIGMLA PM W. W FR/TSCH/ (\1 lNl/E/VTORS .R. 0. SOFFEL us By A. WEAVER A T TORNQ 5 Sheets-Sheet 4 m M FR/TSCI-l/ INVENTORS R. O. SOFFEL I y 4. W54 W57? AT TORNEY June 1953 w w FRITSCHI ETAL VOICE FREQUENCY SIGNALING cmcum Flled May 1, 1952 5 Sh eets$heet 5 w.v wk FRlTSCHl ETAL VOICE FREQUENCY SIGNALING CIRCUIT June 16, 1953 Filed May 1, 1952 Patented June 16, 1953 UNITED STATES PATENT OFFICE VOICE FREQUENCY SIGN ALINGCIKCUIT Walter W. Fritschi, Manhasset, Robert 0. Soflel, Hastings on Hudson, and Allan Weaver, Port- Washington, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. 'Y., a corporation of New York,

Application May 1, 1952-, Serial Nb. 285,354 11 Claims. (01. 179-84) This invention relates to signaling systems and particularly to systems in which signaling is effected by the transmission of alternating currents of voice frequency.

Objects of the invention are the provision of a reliable and stable signaling means in systems employing voice frequency signaling, the prevention of false operation in response to voice currents or other interfering currents and the provision of means to insure signal current impulses of required minimum and maximum duration to insure the required supervisory and switching operations in response to the signals.

The invention is a signaling system in which signals are transmitted by the initiation and the termination of transmission of alternatingcurrent and the transmission of'alternating-current impulses of voice frequency, in which the operation of signal responsive devices is delayed to aid in preventing false operation due to speech currents, and in which the repeated signals are of required duration to effect the desired signal response. I 1 A feature of the invention is a signaling circuit unit arranged to transmit and receive impulses of current of a single voice frequency, and having signal and guard tuned networks combined to-control, throughxseparate electronic tubes, the operation of a first relay and the delayed operae tion of-an impulse repeating relay responsive to incoming signaling current. A further feature is such a signalin circuit unit with adjustable ccupli-ng between the anode of the first tube and grid of a second tubeto obtain a desired delay in the operation of the impulse repeating relay and a desired delay in its release, thereby to insure a minimum length of repeated signal. Another feature is a signaling circuit comprising electronic means controlling the repeating of incoming signal impulses and adjustable capacitor charging means for introducing required impulse correction. A further feature is the provision of means in such a signaling circuit unit for momentarily disablin the impulse repeating relay when a signal impulse is transmitted therefrom.

A clear and complete understanding of. the in,- vention will be facilitated by considering a system embodying the invention and its features, one such system being represented schematically in the drawing of this specification. The invention is not limited inits application to the system and circuit arrangements shown in the drawings but is applicable generally to any system. in which signals-are transmitted by voice frequency alternatingcurrents, a

Referring to thedrawings:

Figs. 1 and 2 represent a first toll office comprising an operator position OPI, an outgoing trunk circuit GT1, a. two-way trunk circuit TWTI, a selector TSI, an incoming trunk circuit ITI', and a signal transmitting and receiving circuit. SCi;

Figs. 4- and 5 represent asecond toll ofiice comprising an operator position 0P2, an outgoing trunk circuit 0T2, a two-Way trunk circuit TWT2, a selector T82, an incoming trunk circuit 1T2 and asignaling circuit SCZ;

Fig. 3A represents a four-wire transmission channel including amplifying repeaters;

Fig. 313 represents a .twowire line including er toll ofiices;-andfor connectingi'ntertoll trunks with switching trunks leading to called manual or automatic ofiices, tocalled toll subscribers lines, to. trunks to community dial offices, and to intertoll. trunks to other toll oflices. Outgoing jacks areconnected to switching trunks, toll subscribers lines, trunks to community dial ofiices and. intertoll. trunks. Answering jacks are connected torecording trunks, toll subscribers lines, trunks from community dial oiiices and intertoll trunks. Intertoll first selectors and intertoll second selectors, if required, are provided for use on calls incoming over intertoll trunks to establish connectionswith other toll, local or community dialoflices. The operators positions are provided with dials. for use in controlling the operationof intertoll selectors in other toll omces and for controlling the operation of selector and connector switches in local or community dial offices. Reference may be made to Patent 2,209,777 to R. E. King and O. R. Miller of July 30, 1940, and 2,306,236 to J E. Walsh of December 22, 1942,

for a complete disclosure of the cord and opera- The two toll ofiices shown in the drawings are v interconnected by either four-wire lines or twowire lines, or a combination of both. A fourwire line is shown in Fig. 3A and comprises two two-wire lines TLI and TL2. A two-wire line is shown in Fig. 3B and comprises the two-wire toll line TL3. As shown this line includes terminal amplifying repeaters; it may however alage path to ground potential to eliminate transients produced by relay operations in the trunk and signaling circuits. The voice amplifier 2H3, which provides only sufi'icient gain to overcome the loss of the blocking network unit 2l0, blocks transmission from hybrid coil H02 to transformer T2! and converts the unbalanced to ground input circuit from transformer T2! to a balanced to ground output required for the trunk circuits so include intermediate amplifyingfrepeaters. ,7

The illustrations of Figs. 3A and 33 are to be understood as indicating that either a four-wire or a two-wire line may be operatively associated with the signaling circuits SCI and S02. In the former case, line TLI serves to transmit voice currents and signaling current of a particular voice frequency from the first toll office to the second and the line TL2 serves to transmit voice currents and signaling currents of the same particular voice frequency from the second toll office to the first. In case of two-wire line operation, the line TL3 serves to transmit voice currents in both directions, to transmit signaling current of one particular voice frequency in one direction, and to transmit signaling current of a different particular voice frequency in the opposite direction. The frequency used for signal- 7 ing over a four-wire line may be 2,600 cycles per second; and the frequencies used for signaling over a two-wire line may be 2,600 cycles per second in one direction and 2,400 cycles per second in the other direction. A hybrid coil I-ICZ individual to trunk circuits OT! and TWT I in the first toll ofiice, and a hybrid coil H04, individual to trunk circuits 0T2 and TWTZ in the second toll office, are ararnged to transmit voice currents ,over the associated two-wire orfour-wire toll line between the two toll ofilces. Balanc ing networks BN are connected tozthese hybrid coils in usual and well-knownmanner. These hybrid coils and balancing networks are shown in Figs. 2 and 4 together with the signaling circuits SCI and SC2.but, since they are separately mounted, do not constitute a part of these signaling circuit units.

The signaling circuit SCI comprises conductors 213i and 202 and conductors and 302 for transmitting voice currentsfrom trunk circuits OT! and TWTI through hybrid coil HCZ over the associated two-wire or four-wire toll line, shown in Fig. 3A or 33; a cut-off relay 204 for opening the connection'between the hybrid coil H02 and the associated toll line for a short, desired interval of time whenever the transmission of signaling current is initiated or terminated;

OT! and TWTi. The network unit 2H] constitutes a band elimination filter, network 2 being antiresonant and network. 2l2 series resonant at the signaling frequency to block the transmission of signaling current through amplifier 2I8, signal repeating being effected by direct current signals as hereinafter described.

The associated toll line of Fig. 3A ,or 3B is further coupled by the right windings of transformer TZI, conductors 2M and 242, condenser 243, potentiometer 2M and resistors 245, 246 and 241 to the grid cathode circuit of an amplifier comprising the triode tube 250, the anode-cathode circuit of which includes the right winding of transformer T23. Immunity to interference from low frequency noise is obtained by providing va negative feedback through condenser 248, the effect of which is increased at low frequencies by the high impedance of the by-pass condenser 249. The resistor 246 in the grid circuit effects volume limiting and a condenser 252 is connected across the output winding of trans- V former T23 to attenuate harmonics of the sigand means comprising a transformer T2], conductors 20? and 208, a network unit 210 consisting of networks 2| I and 2 i 2, a potentiometer 2 I3, an amplifier tube 2? and a transformer T22 for transmitting voice currents from the associated toll line through hybrid coil HCZ to trunk circuits OTI and TWTI. Relays 220 and 230 control the transmission of signaling current from a tone source 080! over the toll line of Fig. 3A or 313, as hereinafter described, relay 220 being operatively controlled over signaling conductor 16 1 by the associated trunk circuit OTI or TWTI The retardcoil 200 provides a continuous drainnaling frequency which may be produced due to the volume limiting action of the grid resistor 246. The resistor 25I, through which positive plate'voltage is applied to the anode of tube 250, together with condenser 249 also limit feedbackof signaling frequency current through the anode voltage source. 7

The signaling circuit SCI further comprises a sharply tuned network unit, 254 consisting of network 255 which is antiresonant at the incoming signal frequency and a network 256 which is series resonant at the incoming signal frequency. The network 255 and the associated rectifier element 253 constitute'a signal channel for effecting desired signal operations, the voltage across network 255 being maximum at signaling frequency; while the network 256 and electronic rectifier consisting of the right triode of tube 210 connected as a diode constitutes a guard channel which prevents false operations in response to voice, noise or other currents containing a component of the signaling frequency. The rectified voltages developed in the signal and guard channels are combined algebraically through resistors 259 and 26i; and. the combined voltage is applied through resistor 215 to the grid of a direct-current amplifiertube 280 to immediately operate control relay 285 in response to signaling current incoming over the associated toll line; and is applied through a delay network, consisting of condenser 267, variable resistor 268, and resistor 260 to the grid of the lefttriode of tube 210, which triode constitutes a direct-current amplifier, to effect the delayed operation of the signal repeating relay 290. The direct-current amplifiers are normally biased almost to cut off by a voltage dividing network comprising a negative voltage source, resistors 218, 271, 219, 213 and 214 and the heating elements of tubes 2 i8, 250, 210 and 286. The biasing connection for the left triode of 210, is traced from the grid through resistors 269, 268, 259, 26I and 266 to the voltage dividing network; and the biasing connection for "the ,tr ioue zo'u is traced from the grid through resistors 2T5, 259-, 201 and 266 to the voltage dividing network? The op eration'or control relay 285 in res onse to signaling current incomrngover the associated toll time, prior to the operation of signal repeating relay 2st, effects the insertion of the band-slime nation-filter 21 in the input to voice amplifier 2I8 to prevent the further transmission'o f the signaling current through the amplifier and as soci'ated trunk circuits thereby to prevent a s'ig nal response in another signaling circuit of a multili'nlr connection so as to limit signal re sponse to a single link, all 's'ignalsincoming over the associated toll line being repeated by relay 290 as direct current signals over-conductor M0 to the trunk circuits OTI and'TWTl, as her'e inafter described. A rel'a' y 296 is controlled by the impulse repeating relay 290 to disable the guard channel a predetermined interval after the operation of relay 290, as hereinafter further described, Although triodes 250 and 280 are shown as being in separate envelopes, it is apparent that both may be within a common envelope. Similarly, although tube 210 is shown as a duotr-iode, it is apparent that each triod'e' may be in a separate envelope. The operation and functionor all circuit elements including those above mentioned are" hereinafter completely described on a call from the first to the second toll office. The signaling circuit S62 is identical in its components, functions "and operations with the signaling circuit SCI, corresponding elements being identified by the same referencech'arac' t'er's except for the first digit thereof.

It may be noted that the various voltage sources for these circuits are grounded, the ungrounded pole being represented in the drawings by a circle within which a negative or a positive' sign indicates the polarity. The oscillators OSCI and 0802 which supply signaling current of desired frequency maybe of. conventional de; sign. It is to be noted that the normally operated relays are shownoperated and .thenormally non-'operatedielays are shown non-operated in the drawings. v

Normally, that is. during- .the times the a'sso-' ciated toll line of Fig. .3A or 3B is idle, signal v ing current is being transmittedthereover.irom each of signaling circuits SCI. and, SC2;,v relays 204, 220 and 230 of circuitS Cl and relays 404, 420 and 430 of circuit SClare not operated, the amplitude of the transmitted signaling current being at low level due to the inclusion o f resistors 2'31 and 232" and 43'! and 432 in" the transmission paths. Iii response to the normally transmitted signaling currents, relays 285 and 290 or signaling circuit SCI and relays 4"85arid 490 or signaling circuit 'SC2 are operated. with raw 29'0 operated, ground is disconnected from signaling-conductor 110; and, with relay '490"0p'er-" ated, ground is disconnected from signaling conductor 4T0. Relays wear-1d 406 are not; oper-'- ated since their windings are short circui-ted by the front contactsot relays zscanaaeo respec= tively;- the guard channel network 2 56 and re:

sistor 258 areshort-circuited by contactsof relay 295; and the guard channel network lifi-a'nd resistor 458 are short-oircuited by contacts of relay 490. With relays 285 and 4-85 operatedand relays 220 and. 420 not operated, the band elimination filters 2:10 and 4I0are inserted'be tween transformers TZI and T22. i i Assume now that a toll operator 'ata positi'o QPI in the firsttollofiice (Fig; 1) .-.=has.a-nswered-.

6'. an inoomifig call either tron a loeal since or from another toll orfiee v'vi ththe answering plug APl' ofacorn: CD1 and that the call is to be extended "to the second toll office (-Fig. 5). The calling plug 'C-Pl is thereupon inserted in a jack J of circuit; 0T1 causing the o iieration of i uit through the s1eeve or signal-relay I40 1 ng' conductor of the f relafy I04 causes the op: oration f relay I I 0, as" described in the aforementioned Walshpatent- The o eration of re Ht disconnects signaling conductor I 10 f-rom 6o dutor 1-09, shy to pro ent operation of I003 of' two-w y TWT I when ground is connected to signal g conductor [1 0 in signaling circuit SCI, and connects conductor [1 0' to the wind-ingorsupervisory relay "I05 of trunk circuit 'QTI. The operation-of relay H0 also connects ground to conductor I68 to 0perate relay i004- or "trunk circuit 'TW'II and to mark t circuit TWT-l as busy in case this trunk circuit-is selectable by tollroute selectors as described in the King et al. and Walsh patents. Relay rout locks to conductor "58' independent of relay I00 3=and further opens the con nection' between the winding of relay I003 and signaling conductor The operation -of ire-"- l'ay 1'40 closes a circuit from 'battery through resistor I42, front contactof relay F40,- conductor 54 and winding of relay 220 of the circuit 'SCI, operating relay2-20. it being fur-=- ther assumed that the-signaling circuits-SCI and S02 are interconnected by the four-wire t'oll line of Fig. 3A, the operation of relay 22'0 disc'on nects oscillator OSCi from conductors 301 and 302 thus terminating the transmission of sig= nali'n'g current over the toll line TLI. eration "of relay 220 also closes a circuit for op erating relay 2'04 and opens a normally closed shunt through resistor' 22-2 across tuned network 256, this shunt being normally of no effect since network 256 is normally Short circuited through a back contact of relay 290 as above mentioned. The operation of relay 2'04 opens the connection between coil H02 and conductors 301' and 302, connects a terminating resistor 205 across conductors 201 and 202-, connects a terminating r'esistor 200 across conductors 301 and 302, and closes a circuit for operating relay 230. The op eration of relay 230 closes a locking circuit independent of relay 204, short circuits resistors 231 and 23-2 to prepare for the transmission of signaling current of high level from oscillator OSCI, and opens the operating circuit of relay 204. Relay 204 is slow in releasing due to the shunting efiect of resistor 203. The release of relay 204 disconnects the terminating resistors and 'recloses the connection between hybrid coil H02 and toll line TLI.

The aforementioned termination of the trans-' mission of signaling current from oscillator OSCI over toll line TLl due to the operation of relay 220 of signaling. circuit SCI constitutes a seizure signal which causes the release of the normally operated relays 485 and 490, of signaling circuit Relay 4B5 releases first and without delay thereby closing a shunt from the grid of the right triode oftube 41-0 through resistor 494 to the source of cathode potential, this shunt being effective when relay 4% is released to cause the release time of relay 490 to more nearly equal its operate delay on interrupted supervisory tone signals whichhave an on time long enough to release rel'ay 496'. The release of relay 4'90 con- The op 7 nects ground to conductor 519, therebycausing the operation of relay 693 of trunk circuit 'IWT2; opens the shortcircuit around the winding of relay 496 causing the operation of this relay; and opens the above-mentioned path fromthe grid of the right triode of tube 419 through resistor 494 so as not to affect the next operation of relay 499. The operation of relay 496 pm: pares the signal receiver SC2 for receiving dial impulses; it connects varistor 493 across resistor 468 to aid in dissipating the charge on condenser 491; opens the short circuit across network 456;

closes a short circuit across. resistor 451 to increase the selectivity of network 454; opens a short circuit across varistor- 463; and closes either a short circuit or a shunt across resistor 494 depending upon whether relay 439 is or is not operated. The operation of relay 496 also connects ground from a back contact of relay 429 to a permanent signal alarm circuit 499.

The aforementioned circuit for operating relay 693 is traced from ground at the back con-tact of relay 499, through conductor 5'19, lower back contact of relay 519 of trunk circuit 0T2,- conductor 569, back contact of relay 694 of trunk circuit TWT2, back contact of relay 691 and winding of relay 693. The operation of relay 693 connects ground to conductor 569 to guard outgoing trunk circuit 0T2 from beingseized on an outgoing call by. an operator or a toll route selector having access thereto. Relay 693 also closes a circuit for operating relay 695, this circuit being traced from ground at the back contact of relay 499, through conductor 519, lower back contact of relay 519, conductor .569, back contact of relay 694, winding of relay 695, uppermost front contact of relay 693, conductor 561, theme in simplex through resistors 521 and 522, back contacts of relay 529, windings of retard coil 569, conductors 561 and 562, inner back contacts of relay 539, conductors 691 and 692, front contacts of relay 693, back contacts of relay 669 of toll selector T82, resistors 661, 662 and 663 and the winding of line relay 651 of this selector. The line rela 651 is thereby operated and so also is relay 695. The operation of relay 695 closes a circuit through resistor 696 for holding relay 693 operated and closes a circuit for operating the slow-to-release relay 691. The operation Of relay 691 also closes a circuit for holding relay 693 operated, opens the operatingcircuit of relay 693 and connects ground to c'onductor'653 of toll selector T82 to hold selector T82, and any additional toll selector through which the connection is further extended in the second toll ofiice, until the connection is released at the first toll office. The toll selector T52 is now ready to respond to dial impulses.

When the toll operator dials the firstdigit of the toll route number, relay 149 of trunk circuit 0T1 is alternately released and reoperated as many times as there are units in the digit dialed. Each release and reoperation of relay 149 causes the release and reoperationof relay 229. The operate and release times of relay 229 are equalized due to the eifect of the potentiomstar 221. The release of relay 229 responsive to the first dial impulse of a train opens the locking circuit of relay 239 and closes a circuit for ope'r: ating relay 294; and the succeeding reoperation of relay 229 at the end of a dial impulse again closes the locking circuit of relay 239 and opens the operating circuit of relay 294. Being slow in releasing, as hereinbefore described, bothof relays 294 and 239 remain operated during the response of relay. 229 to dial impulses. The continued operation of relay 229 at the end of the last impulse of a digit is effective to hold relay 239 operated and causes the release of relay 204.

The release and reoperation of relay 229 responsive to each dial impulse of a digital train alternately connects the tone source OSCl to toll line TM and disconnects this source from toll line TLl whereby high level impulses of tone are transmitted over the toll line, one impulse of tone for eachdial impulse, to the signal receiver S02. The'first tone impulse of a train causes the reoperation of relay 485 and a delayed reoperation of relay 499. Theeffect'of transients due to energization of the signal and guard networks is reduced by the shunt comprising resistor 422 across the guard network 456. The resistor 415 and condenser 416 ;minimiZe -the effectof high frequency currents with respect to relay 485; and resistor 4,69 and condenser 412 minimize the effect of such currents with. respect to relay 499. The operation of relay 495 closes a circuit including resistor 486 for charging condenser 491 due to the voltage drop across the winding of relay 485, this beingprior to operation of relay 499 and without immediate efiect. The operationof relay 495 introduces the band elimination filter unit 419 between transformer T41 and amplifier 418, thereby to stop the transmission of the tone beyond the signaling circuit 8C2, the tone already passed therethroughbeing insuific'ient in duration to effect signal response; so that on built-up connections a signal response in asucceeding link can only be effected by operation of signal repeating relay499. The operation of relay 499 is delayed by the network comprising variable resistor 468 and condenser 461, the resistor being adjusted so that relay 499 is operated by the shortest tone impulse that can be recognized with certainty as an impulse of signaling current. 'When'jrelay 499 operates, it disconnects ground from conductor 519 causing the release of relay 695 of trunk circuit TWTZ and release of relay 651 of tollroute selector T82; and closes a short 'circuit'across thewinding of relay 496. Relay 496 is slow to release and remains operated during the response of relay 499 to dial impulses. 'The operation of relay 499 also opens the connection between condenser 491 and resistor 486 and connects con-denser 491 through a back contact of relay 429,-a front contactof relay 496 and variable resistor 492 to the grid of the right triode of tube 419. When the tone impulse corresponding to the first dial impulse (of adigital train of impulses) ends, relay 485 releases iollowed by 'the release of relay 499; and the release of relay 499 reconnects ground to'cohductor 519 to reoperate relays 695 and 651.: Since it is necessary that relay651 of selector T82 release responsive to each :dial impulse, it is necessary to make certain that relay 499 does not release before a sufficient interval to assure satisfactoryim-pulse repeating over conductor 519. To this-end, condenser 491 discharges through res stor 492 thereby maintaining the grid of the right triode of tube 419 positive to hold relay 499 operated long enough to insure release of relay 651. The variable resistors 469 and 492 are set for tone impulses of median length, for instance pulses at the rate of ten pulses per second and having fifty-five per cent break,.so as to delay the operation andralso the release of relay 499 for the same desired interval of time. If-theincoming tone impulse terminates and "releases'relay 485 before the end of the minimum required out-- put impulse over conductor 519, condenser 49! by virtue of its connection to the anode of tube 489 introduces an immediate increase in the positive potential of the grid of the right triode of tube 419 thereby augmenting the delay effect through resistor 492. Thus, in the case of short duration tone impulses, the operation of relay 499 is maintained after the incoming impulse terminates. Each succeeding impulse of a digital train effects a like operation .andrelease of relays 485 and 499, whereby the dial impulse signals are repeated over conductor 519 to cause the alternate release and reoperation of relays 995 and 651. Relay 691 .isslow to release so as to remain operated during the response of relay .651 to all of the impulses in a digital train. With relay 496 operated, varistor 493 is connected across resistor 468 to dissipate between impulses the energy stored in condenser 461 so as to insure the same delay in operating relay 499 responsive to each tone impulse in a digital train.

The operations of the signal receiver responsive to tone impulses of a digital train are as above described for impulses of median length or shorter than median length. However, if the tone impulses are so long that the charge on condenser 49'! has been substantially dissipated before an incomin'gimpulse terminates and if after aifurther very short interval the next tone impulse begins and relay485 reoperates, the decrease. in potential of. the anode of tube 489 is transmitted through condenser 49!, a front contact of relay 499 and resistor 492 to the grid of the right triode of tube 419 to hasten the release of relay 499 and thereby assure a sufficient inter: val between impulses transmitted over conductor 519 to effect reoperationof relays 695 and 651. To limit the eiTect of this decrease in potential and prevent interference with response of relay 499 to the succeeding tone impulse, an energy dissipating path is closed from the grid of the right triode of tube 419 through the inner lower front contact of relay 499, lowerback contact of relay 485, inner lower back contact of relay 439 and lowermost front contact of relay 496. The

release of relay 499 opens this dissipating path so that condenser 49 is restored to its usual condition at the start of the succeeding incoming tone impulse.

The dialing of additional digits effects the transmission of trains of tone impulses in like manner and completion of the desired connection. Assuming the call is routed through se1ec-- tor TSZ to an incoming trunk circuit 1T2, relay 669 is operated and the simplex circuit is extended through the brushes of selector T82 to the winding of line relay 195 of trunk circuit 1T2. Relay 1'95 operates in series with relay 695 of two-way trunk circuit TWTZ, lighting answering lamp L1. When the call is answered by insertion of a plug of cord CD2 in jack J1, relay '134 is operated as described in the aforementioned.

10 transmission of signaling current over trunk line T-L2 to signaling circuit "SC! as an answering supervisorysignal. The operationof relay 429 causes in succession the operation of relay 494, the operation of relay439 and the release of relay 494. Relay 4-39 loc-ks under control of relay 429, short circuits resistors 43! and 432, opens the short circuit across resistor 433 and transfers the winding of relay 494 from the front to the back contact .of relay 429. With relays 429 and 485 both operated, the band elimination filter M9 is removed from the talking path between transformer T4! and the answering operator. The operation .of relay 429 also opens the short circuit across resistor 423 and opens the shunt through resistor .422 across the guard network 456 thereby increasing the guard actionto insure against false release due to a signaling frequency component ,in the talking current. The termination of the tone signal over line TLZ causes the successive release-of relays 285 and 299 of signaling circuit :SCI and'the operation of relay 296. Both signaling circuits SCI and S02 are now in talking condition, relays 229, 239, 296 of circuit SC! being operated and relays 429, 4-39 and 496 of circuit SC2 being operated, both band elimination filters :2I-9-and 419 being cutout of the connection between the calling and called operators, and .both circuits having the maximum guard actionagainst false response to voice currents having signal frequency components.

The aforementioned insertion of resistor 423 inseries with condenser 49! and resistor 492 due to the operation of relay 429 increases the-release time .of relay 499, this being necessary to compensate for theincreased-guard action (resulting from the opening of the shunt across resistor 42-2) in case a reringsignal is received while an off-hook signal condition exists in signaling circuit $02. For the same reason,the operation of relay 439 inserts resistor 433 in the shunt path from the grid of the right triode of tube 419 which path is .closed when relay 485 releases and before relay 499 releases-after therering signal ends; whereby disconnection of ground from signaling conductor -51-9-fora long enough interval to repeat the rering signal is assured. Such a rering signal gives the answering operator a sig- I nal in the manner described in the aforementioned King et a1. and.Walsh -patents.

Iftheselector TSZ or some succeeding selector encounters an all-trunks-busy condition, or if a called subscriber line is busy, busy signals consisting of alternate off-hook and on-hook signals are received over conductor 564 at the rate of 60 or 1-20 impulses per minute and are repeated by transmitting toneimpulses over trunk TLZ,

causing the alternate operation and release of relays 285 and 299. Each tone impulse thus transmitted by the alternate operation and release of relay 429 is long as compared with dial impulses and, if long enough, relay 296 Will release between successive operations of relay 299. The release of relay 296 short circuits the guard network, opens the connection between condenser 29! through resistor 292 to the grid of the left triode of'tube'219, and inserts resistor 294 in the shunt path from this grid'to the source of negative'potential; whereby the equality between the operate and releasetimes of relay 299 is maintained at the flashing rate and the signals transmitted by relay 299 .over conductor I19 to relay I of trunk circuit OTs! are repeated by this relay .to alternately?extinguish and light the supervisory lamp 'cs at': the calling operators 11 position. Since the negative transient which results from receiving a signal, during the high guard condition'when relay 296 is operated, may be followed by a positive transient which might Otherwise cause a momentary operation of relay 290, the varistor 263 and resistor 264 constitute a low impedance shunt across resistor 266 to dissipate the positive transient. The resistor 264 protects the varistor 263 against overload cu ent therethrough in case of trouble grounds.

If a stop-dial signal is received by signaling circuit $02 over conductor 56d during the establishment of a connection, relay 420 is operated and the signal is repeated as a tone'signal over toll line TL2 to signaling circuit SCI in the same manner as is an answering supervisory signal in the same direction. Thereafter when a startdialing signal is received over conductor 564, the release of relay 420 starts the transmission of tone at a high level effecting the reoperation of relays 285 and 290 of signaling circuit SCI and the release of relay I05, giving the calling operator a start-dial signal. Since dialing may start immediately thereafter, any echo effect due to the transmission of tone at a high level from signaling circuit SC2 over toll line TL2 is suppressed by a negative pulse transmitted from the front contact of relay 320, through condenser 42 5 and resistor 424, and resistor 066 to the source of cathode potential. Thus any echo of the transmitted start-dial signal will not cause a short, false operation of relay 490.

When, on the aforementioned call from an operator in the first toll ofiice completed through trunk circuit 1T2 to a toll operator in the second toll office, the answering operator disconnects, relays I34 and 6 release in succession in the manner described in the aforementioned Kin et al. and Walsh patents. The release of relay EII disconnects battery from conductor 564 releasing relay 420. The release of relay 420 causes in succession the operation of relay 40.4, the release of relay 430 and release of relay 404. The release of relay 420 also reconnects oscillator OSC2 to toll line TL2 to cause the reoperation of relays 285 and 290 of signaling circuit SCI, followed by the release of relay 296. Relay 290 repeats the signal by releasing relay I05, and relay I05 causes th lighting of supervisory lamp CS as a disconnect signal. When the calling operator removes plug CPI from jack J I, relays I04 and I40 release and the trunk circuit OTI is restored to normal as described in the aforementioned King et al. and Walsh patents. The release of relay I disconnects battery from conductor I64 causing the release of relay 220. The release of relay 220 causes the reoperation of relay 204, opens the locking circuit of relay 230, and reconnects oscillator OSCI to transmit a high level tone over toll line TLI to effect the reoperation of relays 585 and 490 of signaling circuit SCZ. The release of relay 230 after a predetermined interval of time reinserts resistors 23I and 232 in the tone transmission path thus reducing the signal to a low, idle indicating level. When slow-to-release relay 696 releases due to the continued operation of relay 490, it short circuits the guard network to prevent the false release of relay 90 responsive to noise or transients on the line during the idle condition. The continued disconnection of ground from signaling conductor 570, due to the reoperation of relay 490 responsive to the disconnect and idle signal, causes the release of relay 605 of trunk circuit 'IWTI and the release of line relay 105 of incoming trunk circuit 1T2. The release of relay 605 causes the release of relays 603 and 604; the release of relay 60! disconnects ground from conductor 653 thereby restoring selector TS2 to normal; and the release of relay 603 disconnects ground from conductor 568 thereby markingthe trunk circuit OTI idle; all of these operations being realized in the manner described in the aforementioned King et al. and Walsh patents.

It may be noted at this time that the release of relay 490 and operation ofrelay 395 while relay 420 is not operated closes a circuit to energize the permanent signal alarm apparatus 499; so that if a large number of signaling channels are simultaneously disabled, for instance by a cable failure, the permanent signal alarm'circuit would immediately give an indication of such a condition.

If the toll line individually associated with trunk circuits OTI and GT2 in the two toll offices, is a two-wire toll line TL3 as shown in Fig. 3B, the frequency used for signaling in one direction is different from the frequency used for signaling in the opposite direction. In this case, hybrid coils HC3I and H032 provide the outgoing and incoming transmission paths between toll line TL3 and the two signaling circuits SCI and SC2 and there is a blocking filter at each end of the line in the incoming transmission path tuned to block currents of the outgoing signaling fre-. quency so that unattenuated outgoing signals will not interfere with signal operation in response to attenuated incoming signals. Thus the blocking filter 356 which isinserted in the incoming transmission path between line TL3 and transformer TZI of signaling circuit SC'I is tuned to the same frequency as oscillator OSCI. of signaling circuit SCI and blocking filter 316 which is inserted in the incoming transmission path between line TL3 and. transformer TM of: signaling circuit S02 is, tuned to the same fre-' quecy as oscillatorOSCZ of signaling circuitSCZ. The filter 356 comprises networks 351 and 358 which are antiresonant and network 359 which is series resonant at the outgoing signaling frequency from signaling circuit SCI. comprises similar networks 311,310 and 319 tuned to the outgoing signaling frequency from signaling circuit SCZ. A relay 368 controls theinsertion of the filter 356 and a relay 388. controls the insertion of filter 316 filters during talking.

. On a call from the second toll office to first toll office, the operation of the various trunk and signaling circuits is similar to that of the above-described call from the toll ofiice.

It is further to be noted that the signal transmitting relays 220, 420 and signal responsive relays 285; 290, 485 and iflfiare ordinary flat type relays, a common rubber'snubbing means being provided to substantially eliminate vibratory signal distortion due to dynamic weakness in the relay structure. 7

What is claimed is: 1 I

1. In a signaling system in which current of a particular frequency within the voice band is used to effecttransmission of signals, a signal receiver comprising means for amplifying in- .coming currents,signal and guard tuned networks connected in the amplifier output circuit,

first to the second the signal network being antiresonant and the guard network series resonant at the signal frequency, means idividual to each of said networks for separately rectifying the voltages across said Filter 316 to remove the blocking the 13 networks, two signal'respons'ive relaysl'electronici means individual to and controllingthe operation of each of said relays, and means for combining said rectified voltages and applying the resultantreceiveri comprising means for amplifying incoming currents, signal and guard tuned networks connected in the amplifier output-circuit the.- signal. network being antiresonant and 'the guard network seriesresonant at the signal frequency, means individual to each of said networks for separately rectifying the voltages across said networks; two signal responsive relays, electronic means individual-to and controlling the operation of each of said relays, means for combining said rectified voltages and applying the resultant voltage to both of said electronic means, and means for delaying the application of said resultant voltage to only one of said electronic means thereby to delay the operation of said one of said relays.

3. In a signaling system in which current of a particular frequency within the voice band is used to effect transmission of signals, a line over which voice currents and signaling currents of said frequency are transmitted, a signaling circuit connected to said line, said circuit comprising means for amplifying said voice and signaling currents incoming over said line, a signal network and a guard network in series coupled to the amplifier output circuit, said signal network antiresonant and said guard network series resonant at said signaling frequency, means including a rectifier and a first resistor for rectifying the voltage developed across said signal network, means including a rectifier and a second resistor for rectifying the voltage developed across said guard network, first and second relays, a first electronic valve controlling the operation of said first relay, a second electronic valve controlling the operation of said second relay, means comprising said second relay for repeating signals incoming to said circuit, means including said first and second resistors in'series for combining the rectified signal and guard voltages and applying the combined voltage to the control elements of each of said valves, and means including a source of potential and a third resistor connected in series with said first and second resistors for biasing said valves sufficiently to prevent the operation of each of said relays when no voltage is developed across either of said first and second resistors, said combined voltage being ineffective to cause the operation of said relays responsive to voice current incoming over said line and effective to cause the operation of said relays responsive to signaling current incoming over said line.

4. In a signaling system'and signaling circuit according to claim 3, means comprising a variable resistor and a condenser in series connected in parallel with said first resistor for delaying the application of rectified signal voltage to the control element of the thermionic valve controlling the operation of said second relay thereby to delay the operation of said second relay.

5. In a signaling system and signaling circuit according to claim 3, means comprising a variable resistor and a condenser in series connected in parallel with said first resistor for delaying the application of rectified signal voltage to the control element of the thermionic valve controlling the operation of said second relay thereby to derelease third: relay, means rendered effective by release of said second relay for operating said:

thirdfrelay, and. means comprising a varist'or connected across said variable resistorwhen said" third relay is operated to dissipate the voltage across said condenser during intervals between impulses: of signalingcurrent incoming to said circuit.

6. In a signaling system and signalingcircuit" according. to claim 3, a slow-to-rel'ease third relay, means rendered effective by relea'se'of said second relay for operating said third relay, a

condenseroonhected tothe anode of said first thermionic valve, means for charging said concomingsignallng current, and means comprising said condenser and a variable resistor connected in series by said third relay while operated to the control element of said second valve for delaying the decay of the rectified voltage applied to the control element of said second valve upon termination of incoming signaling current thereby to delay the release of said second relay for a desired interval of time.

'7. In a signaling system and signaling circuit according to claim 3, a slow-to-release third relay, means rendered effective by release of said second relay for operating said third relay, a condenser connected to the anode of said first thermionic valve, means for charging said condenser while said first relay is operated and before said second relay operates in response to incoming signaling current, means comprising said condenser and a variable resistor connected in series by said third relay while operated to the control element of said second valve for delaying the decay of the rectified voltage applied to the control element of said second valve upon termination of incoming signaling current thereby to delay the release of said second relay for a, desired interval of time, means comprising a fourth relay effective when said relay is not operated to transmit signaling current from said circuit over' said line, a resistor connected by a normally closed contact of said fourth relay across said guard network to reduce the effectiveness of said guard network, and resistor means inserted by the operation of said fourth relay in said series connection between said condenser and control element for reducing the delay in releasing said second relay upon termination of signaling current incoming to said circuit.

8. In a signaling system and signaling circuit according to claim 3, means comprising a variable resistor and condenser in series connected in parallel with said first resistor for delaying the application of rectified signal voltage to the control element of said second thermionic valve thereby to delay the operation of said second relay, a slow-to-release third relay, means rendered effective by release of said second relay for operating said third relay, means comprising a condenser and variable resistance connected by said third relay when operated in series between the anode of said first valve and the control element of said second valve so as to delay the decay of the signal Voltage applied to said element and thereby delay the release of said second relay.

'9. In a signaling system and signaling circuit according to claim 3, a slow-to-release third relay, means rendered effective by release of said second relay for operating said third relay,

and means comprising an auxiliary biasing path for the control element of said second valve, said auxiliary path closed by release of said first relay upon termination of incoming signal ing current while said third relay is operated until said second relay releases thereby to hasten the release of said second relay.

10. In a signaling system and signaling circuit according to claim 3, a slow-to-releasethird relay, means rendered effective by release of said second relay for operating said third re lay, means'comprising an auxiliary biasing path for the control element of said second Valve, said auxiliary path closed by release of said first relay upon termination of incoming signaling current While said third relay is operated until said second relay releases thereby to hasten the release of said second relay, and additional means comprising a resistor inserted in said auxiliary biasing path in case said third relay is not operated at the time said first relay releases for decreasing the effect of said auxiliary biasing path. a. f V

11. In asignaling systemand signaling circuit according'to claim '3, means comprising a third relay and including normally closed contacts thereon for transmitting signaling current from said circuit over said line, and means comprising a condenser and resistor connected in,

series with said third resistor rendered effective by release of said third relay to increase the efiect of said biasing resistor. I

WALTER W. FRITSCHIL ROBERT O. SOFFEL. ALLAN WEAVER.

No references cited. 

